All we have done above are three phase faults, you may ask, how about single phase faults?
For single phase faults, positive sequence, negative sequence and zero sequence impedances need to be calculated.
If = 3 (I1 + I2 + I0)
Examining the circuit in Figure 1, at the 400V Bus, on Transformer 3 contributes to the zero sequence current.
For transformers, the negative sequence and zero sequence impedance are equal to the positive sequence impedance.
Z1 = Z2 = Z0 or
MVA1 = MVA2 = MVA0
At the 400V Bus
1 / MVAsc =1/3 (1 / MVAsc1 + 1 / MVAsc2 + 1 / MVAsc0)
1 / MVAsc = 1/3 (1 / 28.28 + 1 / 28.28 + 1 / 33.33 )
MVAsc =3 x 9.93 = 29.79 MVA
If = 29.79 / (1.732 x 0.4) = 43 kA
At 6.6KV Bus
1 / MVAsc = 1/3 (1 / MVAsc1 + 1 / MVAsc2 + 1 / MVAsc0)
1 / MVAsc = 1/3 (1 / 91.17 + 1 / 91.17 + 1 / 71.43)
MVAsc = 3 x 27.83 = 83.49 MVA
If = 83.49 / (1.732 x 6.6) = 7.26 kA
Conclusion:
This example illustrates that using the MVA Method of Short Circuit Calculation, it will be very easy to calculate the fault current at any node within a power system.
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thanks a lot and more power & blessings
how do you know that MVAsc utility is 250 MVA ?
@dian,
the 250MVA in this example is just assumed. Ask your utility for the actual fault level at your point of connection
Sir,
If my system is 4.16kv, is there any formula for BIL so I can specify the right cable BIL rating?
BIL is normally 3-4 times the KV rating of an equipment. Refer to standard to ensure you are using the right rating.
Can u pls put some details documents for eath and unearth cable.
can this method account for NGR grounded system say at 6.6kv level?
@rrphatak
.
Yes you can. The NGR is a resistor so it’s MVA will be
But remember that it does not contribute to the fault current but instead limits it.
Treatment of Resistor will be like a transmission line?
R will be dominant in the total Z.
So %/ MVA will be Z(R).100%/ Un^2.
In case of a single phase fault i shall add Z1+Z2 +Zo +3 * %/MVA of the NGR.
Is this treatment right?
paano po i compute ang MVAsc ng GEnerator at Motor 1
@norato:
Check these equations.
Hi, Mr Ver,
I have a doubt, how did you get MVAsc1 and MVAsc2 at 400V and 6.6kV?
Thanks for excellent work.
@Larroya, the MVAsc of each branch is inversely proportional to the MVAsc of each branch in the opposite direction.
Hello mr Ver
i want to asked about MVAsc Utility, can you sharing how to calculated ?
if i had different voltage little bit higher in 20kV, 66 kV or 115 kV, how much i must put MVAsc ?
@riza,
please refer to this article.
Thanks
thank you Mr Ver..
How did u calculate Downstream for generator. I can’t find out any supportive calculation for 133.32MVA as u mentioned in generator downstream calculated value.
@shozy
That is the sum of all currents towards the 11 kV bus of the generator (76.87+27.11+29.41)